Hardwoods constitute over one-third of the U.S. timber resource. However, with the exception of cross-ties, hardwoods are rarely treated for exterior use applications, and demand for treated hardwood products has until recently been low. In view of the projected softwood timber shortage and relative abundance of hardwoods, expanded use of treated hardwoods for both composite and solid wood products is expected. The problem is that replacing softwoods with hardwoods is not straightforward since most wood preservatives, including second generation biocides, are considerably less effective when used to treat hardwoods. (Nicholas, D. D., Proc. of the Northern Hardwood Resource: Management and Potential Conference, Houghton, Mich., Aug. 18-20 (1986); Preston, A. F., et al., Proc. Am. Wood Preservers' Assn., 79, 207 (1983)). This disparity is attributable to the considerably higher toxic threshold values obtained when treated hardwoods are attacked by white- and soft-rot fungi as compared to softwoods treated with the same biocide and exposed to brown-rot fungi. (Nicholas, supra.) Accordingly, the object of the present invention is to provide wood preservative systems that are effective in protecting hardwoods and providing greater efficacy for softwoods.
An apparent solution to the problem of preserving hardwoods is to use substantially higher biocide levels, but this approach leads to higher costs and increased environmental risks. A more attractive solution would be to increase the efficacy of biocides for treating hardwoods.
In addition, more environmentally benign preservatives to treat softwoods are needed, since all major wood preservatives used today to protect softwoods have perceived environmental problems.